1. Field of the Invention
The present invention relates to immersion lithography, and more particularly, to a fluid confinement plate on a lithography tool positioned opposite to an imaging surface of a substrate so that the substrate is submerged in the immersion fluid during exposure.
2. Related Art
A typical lithography tool includes a radiation source, a projection optical system, and a substrate stage to support and move a substrate to be imaged. A radiation-sensitive material, such as resist, is coated onto the substrate surface prior to placement onto the substrate stage. During operation, radiation energy from the radiation source is used to project an image defined by an imaging element through the projection optical system onto the substrate. The projection optical system typically includes a number of lenses. The lens or optical element closest to the substrate is often referred to as the “last” or “final” optical element.
The projection area during an exposure is typically much smaller than the imaging surface of the substrate. The substrate therefore has to be moved relative to the projection optical system to pattern the entire surface. In the semiconductor industry, two types of lithography tools are commonly used. With so-called “step and repeat” tools, the entire image pattern is projected at once in a single exposure onto a target area of the substrate. After the exposure, the wafer is moved or “stepped” in the X and/or Y direction and a new target area is exposed. This step and repeat process is performed over and over until the entire substrate surface is exposed. With scanning type lithography tools, the target area is exposed in a continuous or “scanning” motion. The imaging element is moved in one direction while the substrate is moved in either the same or the opposite direction during exposure. The substrate is then moved in the X and/or Y direction to the next scan target area. This process is repeated until all the desired areas on the substrate have all been exposed.
It should be noted that lithography tools are typically used to image or pattern semiconductor wafers and flat panel displays. The term substrate as used herein is intended to generically mean any work piece that can be patterned, including, but not limited to, semiconductor wafers and flat panel displays.
Immersion lithography systems use a layer of fluid that fills a gap between the final optical element of the projection optical system and the substrate. The fluid enhances the resolution of the system by enabling exposures with numerical apertures (NA) greater than one, which is the theoretical limit for conventional “dry” lithography. The fluid in the gap permits the exposure with radiation that would otherwise be completely internally reflected at the optical-air interface. With immersion lithography, numerical apertures as high as the index of refraction of the fluid are possible. Immersion also increases the depth of focus for a given NA, which is the tolerable error in the vertical position of the substrate, compared to a conventional dry lithography system. Immersion lithography thus has the ability to provide greater resolution than can be performed using conventional dry lithography.
In immersion systems, the fluid essentially becomes part of the optical system of the lithography tool. The optical properties of the fluid therefore must be carefully controlled. The optical properties of the fluid are influenced by the composition of the fluid, temperature, the absence or presence of gas bubbles, and out-gassing from the resist on the wafer.
One known way of maintaining the immersion fluid in the gap where exposure of the substrate is to occur is the use of an air curtain. With an air curtain design, an immersion element, with air jets, surrounds the last optical element of the projection optical system. The air jets are used to create a curtain of air surrounding the exposure area, maintaining the fluid localized within the gap under the last optical element. A number of disadvantages are associated with air curtain type immersion lithography tools. The air jets tend to cause a relatively large amount of evaporation of the immersion fluid at the air-fluid interface. As the fluid evaporates, it may cause the surface temperature of the substrate to cool, causing localized deformation of the imaging surface. The evaporation can also cause contaminants to be left behind on the surface of the substrate. The air jets also have a tendency to create air bubbles in the immersion fluid. Contaminants, air bubbles and surface deformation may each cause overlay and printing errors resulting in defects. For more information on air curtain type immersion tools, see for example U.S. Patent publication 2005/0007569 or European Patent Applications EP 1 477 856 A1 and EP 1 420 298 A2, incorporated by reference herein for all purposes.
Another known way of maintaining the immersion fluid within the gap of a lithography tool is with the use of a nozzle that surrounds the last optical element immediately above the area to be exposed on the substrate. The nozzle includes one or more fluid inlets that introduce the immersion fluid into the gap. The nozzle may also include one or more porous members, pulling, for example, a vacuum below the “bubble point” of the porous material; through which the immersion fluid is recovered. For more information on nozzle type immersion lithography tools, see U.S. application Ser. No. 11/362,833, and PCT Application Serial Numbers. PCT/US2004/22915 and PCT/US2005/14200, all incorporated herein by reference for all purposes.
It is also known to maintain the immersion fluid in the gap between the last optical element and the imaging surface of the substrate by immersing both in a container filled with immersion fluid. The problem with this approach is that during stepping or scanning, the substrate table holding the substrate has to be moved, often at rapid speeds, causing the immersion fluid to “slosh” around within the container. The sloshing fluid causes the projection optical system to mechanically vibrate, inducing overlay and printing errors. Fluid may also be displaced from the container. See for example U.S. Pat. No. 4,509,852, also incorporated by reference herein.
With immersion lithography, any fluid droplets on the substrate are undesirable. If the droplets dry or evaporate on the substrate, any contaminants in the droplet, for example residues dissolved from the resist, remain deposited on the wafer. These deposits may cause printing errors. In addition, if droplets later join the main body of immersion fluid due to the movement of the substrate, the droplets could entrap air which can cause bubbles in the main body of the fluid.